Signs of a weak El Nino

John H Cushman Jr, October 8, 2012, New York Times News Service

A season of warmer ocean waters that has been expected to produce an El Nino episode and perhaps bring relief from the continuing drought may turn out to be a bit weaker than advertised, according to climate experts.

The periodic upwelling of warmer waters in the eastern equatorial Pacific can be one of the most telling calls that a climatologist can make. A powerful El Nino can drive global patterns of drought, storm, snow and flood, with big consequences for farmers and fishermen, relief organisations and reservoir operators, schoolchildren and ski bums. But after seeing signals for months that a moderate El Nino might be arriving right about now, the more likely case appears to be an episode that is weak indeed: probably short, and hardly nasty or brutish. Scientists who predict the weather months in advance pay close attention to back-and-forth swings in what they call El Nino Southern Oscillation, or ENSO, which includes the mirror-image oceanic cooling called La Nina that probably made the past year’s drought worse.

Debatable strength

“We believe that there will be an El Nino, but the strength of it is debatable, and it may be a fairly weak one,” said Huug van den Dool, a meteorologist at the federal government’s Climate Prediction Center.

“The bigger the El Nino, the bigger the effect,” said David Neelin, a professor of atmospheric and oceanic science at UCLA. “This may be El Nino manque, a borderline El Nino – a wannabe El Nino.” That is disappointing news for anyone hoping for much relief from the drought that has gripped the United States. Some people thought a wetter winter might be in store across the US’s midsection when they heard, starting in the spring, that the latest La Nina had ended and that El Nino was on its way. It is now starting to look as though wetter weather in the next few months will be more likely along the Gulf Coast and in the Southeast than in the heart of the drought zone. A weak to moderate El Nino can also favour snow along the Northeastern corridor through Baltimore, New York and Boston, where the wet pattern could meet up with normal freezing temperatures. After midwinter, the models diverge but suggest that any El Nino will wane quickly.

The consensus of the world’s weather gurus remains that “a weak El Nino may develop in October and last until the Northern Hemisphere winter,” the World Meteorological Organization said recently. “There are times, and this year is one of them, when it is more subjective,” said Mike Halpert, deputy director of the Climate Prediction Center at the National Oceanographic and Atmospheric Administration. It takes three things for his agency to formally declare the arrival of El Nino. One is that equatorial sea surface temperatures rise to at least half a degree Celsius above normal across a particular part of the Pacific. A second condition is that the atmosphere must show, in measurements of tropical winds and moisture, that it has plugged in to the ocean’s energy and is beginning to respond. Here the signals have been even weaker.

There is another uncertainty: What the scientists are watching for are anomalies, or deviations from normal averages. But with climate change, the definition of normal has itself been shifting.